1. Field of the Invention
The present invention concerns a device for removing water droplets, ice grains, frosted or condensated moistures, etc. deposited to plate-like members, for example, a reflection mirror of an automobile.
2. Description of the Prior Art
The device of the aforementioned type has been disclosed, for example, in Japanese Utility Model Laid-Open Sho 61-30552 (Date of Laid-open: Feb. 24, 1986) and Japanese Patent Laid-Open Sho 59-8548 (Date of Laid-open: Jan. 17, 1984).
Japanese Utility Model Laid-Open Sho 61-30552 shows a device for removing water droplets deposited on an automobile mirror disposed to the outside of a car body.
As shown in FIG. 21, a back mirror disposed to the outside of a car body comprises a back mirror main body 102 mounted with a mirror 101 made of glass and supported by a stay 103, in which vibrator 104, for example, made of ceramics is disposed between the back mirror main body 102 and the stay 103. The supersonic vibrator 104 is connected with an operation switch 105 disposed inside of a passenger's chamber so that the vibrator can be operated from the inside of the car. Further, a driving circuit 106 and a power source 107 are connected in series between the ultrasonic vibrator 104 and the operation switch 105. The driving circuit 106 is so adapted to amplify a signal generated from an oscillator by an amplifier and send the amplified signal to the ultrasonic vibrator 104 to cause vibration the latter. The device for removing water droplets on the automobile mirror using the conventional ultrasonic vibrator having such a constitution can operate as described below.
If water droplets, etc. are deposited to the surface of a back mirror, the operation switch 105 in a car chamber is operated to vibrate the ultrasonic vibrator 104. The back mirror main body 102 is entirely vibrated by the vibration of the ultrasonic vibrator 104 and the water droplets deposited to the mirror surface can be removed.
However, for uniformly removing the water droplets, etc. deposited on the surface of the mirror 101 by the ultrasonic vibrator 104, it is necessary to vibrate the mirror surface with no unevenness. However, since the ultrasonic vibrator 104 vibrates along the direction of the thickness in the conventional device, it is necessary that the ultrasonic vibrator 104 has substantially the same size as that for the mirror 101, in order to vibrate the mirror surface entirely. As the shape of the ultrasonic vibrator 104 is enlarged, the ultrasonic vibrator 104 tends to cause uneven vibration. Then, if the ultrasonic vibrator 104 is made substantially identical, in the size, with that for the mirror 101, composite resonance is caused between the ultrasonic vibrator 104 and the mirror 101, to generate great stress or heat to the ultrasonic vibrator 104 or the mirror 101 in view of the balance between the size and the thickness of the mirror 101 and that of the ultrasonic vibrator 104, to possibly result in cracking to the ultrasonic vibrator 104 and the mirror 101. In particular, when bending vibrations due to the lateral effect is transmitted to the mirror 101, if the diameter of the ultrasonic vibrator 104 is large, the inner stress in the mirror 101 is extremely increased locally, possibly leading to the destruction of the mirror 101.